The present invention relates to creating highly uniform magnetic fields in magnets used for magnetic resonance imaging, by the use of passive shimming to improve magnet field homogeneity.
To create a highly uniform magnetic field with an electromagnet or array of permanent magnets, it is necessary to build the magnet to a carefully specified shape, and to strive to minimize the deviations from the specified shape due to manufacturing variations. The resulting magnets, however, typically require field corrections to achieve the desired level of inhomogeneity, due to deviations of the magnet from the design or due to the presence of ferromagnetic material in the vicinity of the magnet.
To improve field uniformity, correction coils are typically used. These coils are capable of creating different field shapes which can be superimposed on an inhomogeneous main magnetic field to perturb the main magnetic field in a manner which increases the overall field uniformity. Unfortunately, many sets of such coils are typically required. A state of the art magnetic resonance (MR) imaging magnet has between ten and twenty independent sets of correction coils, each with its own power supply to provide the correct current flow. Naturally, these coils add significantly to the cost and complexity of the magnet.
Passive shimming is currently used to correct large deviations in magnetic fields that cannot be corrected by the available correction coils alone. The passive shimming is accomplished by placing a piece of iron in an appropriate place outside the magnet. The desired level of field uniformity can then be achieved by the correction coils.
It is an object of the present invention to provide a passive shimming assembly for a magnetic resonance magnet which can achieve a level of field inhomogeneity required for magnetic resonance imaging, without the use of correction coils.
It is a further object of the present invention to provide a shimming assembly which when used with a superconducting magnet can be adjusted without de-energizing the magnet.